Hypoxia can induce erythropoiesis through regulated increase of erythropoietin production. We investigated the direct influence of oxygen tension on erythroid progenitor cell differentiation using cultures of adult human hematopoietic progenitor cells exposed to decreasing oxygen tension and independent of variation in erythropoietin levels. Decreases in hemoglobin-containing cells were observed with decreasing oxygen tension). This is due, in part, to a reduction in cell growth and, at low oxygen tension, a marked increase in cell toxicity. Cells were characterized by an early induction of gamma-globin expression and a delay and reduction in peak levels of beta-globin expression at reduced oxygen. Overall, fetal hemoglobin and gamma-globin expression were increased at physiologic oxygen tension, but these increases were reduced at low oxygen as cultures become cytotoxic. At reduced oxygen, erythropoietin induction of its receptor was decreased and delayed, analogous to the delay in beta-globin induction. The oxygen-dependent reduction of the erythropoietin receptor can account for the associated cytotoxicity at low oxygen. Epo induction of erythroid transcription factors, EKLF, GATA-1, and SCL/Tal-1, was also delayed and decreased, consistent with lower levels of erythropoietin receptor and resultant erythropoietin signaling. These changes in globin gene expression raise the possibility that the early increase of gamma-globin is a consequence of reduced erythropoietin signaling and a delay in induction of erythroid transcription factors.